1. Field of the Invention
The present invention relates to a rolling bearing having self-lubricating and a retainer using therefor.
2. Description of the Related Art
When used in an environment where a clean atmosphere is required, such as clean room, semiconductor producing apparatus, liquid crystal panel producing apparatus and hard disc producing apparatus and in a liquid or an environment where a mist or spray of liquid is present, such as various cleaning apparatus and food processing machines, a rolling bearing cannot be lubricated by any lubricant or grease. Therefore, a self-lubricating rolling bearing which doesn""t need to be lubricated by lubricant or grease has heretofore been proposed.
As such a self-lubricating rolling bearing, there is disclosed in Japanese Patent No. 2,709,119 a rolling bearing comprising inner and outer rings made of stainless steel, a retainer prepared only by forming a tetrafluoroethylene-ethylene copolymer (ETFE) comprising a potassium titanate whisker short fiber having a diameter of not more than 2 xcexcm incorporated therein and a rolling body made of hard carbon. This bearing is arranged such that when the retainer and the rolling body come in frictional contact with each other, ETFE constituting the retainer is partially transferred to the rolling body and the inner and outer rings to form a thin film of ETFE thereon.
JP-A-4-331819 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined Japanese patent publication (kokai)xe2x80x9d) discloses that a retainer is formed by a perfluoroalkoxy resin (PFA) comprising a heat-resistant fiber incorporated therein.
The retainers disclosed in the above cited publications can be produced by melt molding and thus have a higher productivity than retainers made of polytetrafluoroethylene (PTFE), which cannot be melt-molded. However, since ETFE and PFA have a relatively low elastic modulus, retainers made of these materials are liable to deformation during use under a high load or at high rotary speed. Further, these retainers can easily generate heat at contact points due to friction. This can result in the locking of the bearing or the development of abnormal abrasion in the retainers. Accordingly, rolling bearings comprising the retainers disclosed in the above cited patents may exhibit a shortened life under a high load or at high rotary speed.
Further, for the production of these retainers made of FTFE or PFA, the mold temperature or resin temperature at which these materials are melt-molded needs to be raised because these materials have a high melting point (melting point of ETFE: 260xc2x0 C.-270xc2x0 C.; melting point of PFA: 300xc2x0 C.-310xc2x0 C.). The higher the resin temperature is, the more easily can be produced a corrosive fluorine-based gas. The heat thus generated and the corrosion by fluorine-based gas shorten the life of the mold.
A rolling bearing comprises an inner ring having a groove on the outer periphery thereof, an outer ring having a groove on the inner wall thereof, a plurality of rolling bodies rollably disposed between the groove on the inner ring and the outer ring, and a retainer in which the plurality of rolling bodies are retained in such an arrangement that they are guided through the gap between the groove on the inner ring and the outer ring.
In general, a rolling bearing is lubricated by circulative supply of lubricant or grease into the bearing or incorporation of lubricant or grease in the interior of the bearing. However, when operated at elevated temperatures or in vacuum, a rolling bearing is liable to the scattering of lubricant or grease to the exterior of the bearing or the evaporation of lubricant or grease accompanied by the release of gas that can stain the external atmosphere of the bearing. Thus, no lubricants or greases can be used in the case where a pure atmosphere is required, e.g., in clean room, semiconductor element producing apparatus, liquid crystal panel producing apparatus, hard disc producing apparatus, and in other atmospheres, e.g., at elevated temperatures, in vacuum, in a special atmosphere, at extremely low temperatures, under irradiation.
Thus, as a method for lubricating rolling bearing there has heretofore been proposed a method free from lubricant or grease. For example, JP-A-55-57717 and JP-A-61-55410 disclose an approach involving the formation of a thin lubricating film made of molybdenum disulfide, graphite, silver, lead or the like on a part or the whole of the surface of inner ring, outer ring, retainer and rolling body by sputtering, ion plating, sintering, etc.
Further, JP-A-62-151539 and JP-A-64-79418 disclose the formation of a rolling bearing retainer by a sintered alloy obtained by sintering a material having a metal powder mixed with a solid lubricant such as molybdenum disulfide, graphite and hexagonal boron nitride. Moreover, JP-A-2-245514 and JP-A-4-102718 disclose an approach involving the formation of a rolling bearing retainer by a material comprising a resin composition having a solid lubricant incorporated in a plastic. In these rolling bearings, when the retainer and the rolling body come in frictional contact with each other, the material constituting the retainer is somewhat transferred to the rolling body, inner ring and outer ring to form thereon a thin lubricating film of the solid lubricant contained in the retainer material.
With the enhancement of the properties of devices and the production yield thereof, the temperature at which these devices are produced has been raised more and more, and the pressure under which these devices are produced has been reduced more and more. Under these circumstances, there is a growing demand for rolling bearings which can operate while scattering little particles to the exterior thereof in a severe atmosphere, e.g., at higher temperatures or under higher vacuum, over an extended period of time.
However, the approaches disclosed in these patents have the following disadvantages.
In other words, the approaches disclosed in JP-A-55-57717 and JP-A-61-55410 are disadvantageous in that the frictional force developed on the contact surface upon the operation of the bearing causes the thin lubricating film which has previously been formed on the surface of the grooves to be abraded to disappearance. The disappearance of the lubricating film causes the elimination of the lubricating effect. This results in the cohesion and seizing of the base metal (matrix). In general, accordingly, a rolling bearing which is thus lubricated can hardly operate over an extended period of time.
The retainer made of a sintered metal disclosed in JP-A-62-151539 and JP-A-64-79418 is disadvantageous in that it is very expensive. This is because a sintered metal cannot be formed into a complicated shape by molding and a rod or ring molded product of sintered metal must be machined to obtain a retainer having a desired shape. Further, a sintered metal normally exhibits an excellent heat resistance but shows a low mechanical strength and is brittle. Thus, a sintered metal can be cracked on the surface thereof when machined. As a result, a sintered metal must be slowly machined and thus shows a poor machinability.
Moreover, the approaches disclosed in JP-A-2-245514 and JP-A-4-102718 are disadvantageous in that if the plastic used as matrix of the retainer material has an insufficient lubricity, its lubricating effect is lost early during the operation of the bearing, causing rapid rise in the generation of abrasion particles. As a result, a large amount of particles are scattered from the bearing to the exterior of the bearing. Further, the resulting torque rise can terminate the life of the bearing. As the plastic having a high lubricity employable as matrix there may be used polytetrafluoroethylene (PFTE) resin. However, this resin cannot be melt-molded (melt molding is a method which comprises heating a resin to a temperature higher than its melting point, and then allowing the fluid resin to be cured in a desired shape in a mold) and thus needs to be machined to form a retainer similarly to the sintered metal as mentioned above. As a result, the production cost is raised. Further, if the plastic to be used as matrix of the retainer has an insufficient heat resistance, the rolling bearing exhibits remarkably deteriorated abrasion resistance or lubricity when operated at high temperatures. Thus, the generation of abrasion particles shows a rapid increase. Accordingly, a large amount of particles are scattered to the exterior of the bearing. Further, the resulting torque rise can terminate the life of the bearing.
In general, a melt-moldable plastic has a relatively low glass transition point or melting point and thus can hardly exhibit an improved heat resistance in a high temperature atmosphere. On the contrary, a plastic having an excellent heat resistance cannot be heat-molded. Thus, heat resistance and moldability cannot stand together. A thermoplastic polyimide, polyether nitrile or the like is a plastic which exhibits an excellent heat resistance but can be melt-molded. However, since such a plastic is relatively poor in fluidity, it is liable to cracking in the tip or curved portion such as lip for holding rolling bodies during the melt molding of retainer. Thus, such a plastic leaves something to be desired in moldability.
It is an object of the present invention to prolong the life of a rolling bearing which is rendered self-lubricating when a synthetic resin material constituting a retainer is partially transferred thereto under a high load or upon high speed rotation as well as prolong the life of the mold for retainer so as to enhancing the mass-producibility of the retainer.
It is another object of the present invention to provide a rolling bearing having a retainer formed by a material containing a solid lubricant incorporated therein which can be kept fairly lubricated scattering little particles to the exterior of the bearing even in a high temperature atmosphere over an extended period of time and can be produced at low cost.
According to a first aspect of the present invention, a retainer constituting a rolling bearing obtained by melt-molding a polyvinylidene fluoride resin (PVdF) or a resin composition comprising a polyvinylidene fluoride resin as a main component. The chemical formula of PVdF is xe2x80x94(CH2CF2)nxe2x80x94.
According to a second aspect of the present invention, a rolling bearing comprises the retainer according to the first aspect.
The retainer according to the first aspect is formed by a PVdF, which has a high elastic modulus than ETFE and PFA, or a resin composition containing PVdF as a main component. Thus, when operated incorporated in a rolling bearing under a high load or at a high rotary speed, the retainer according to the first aspect is less liable to deformation and generates less heat upon friction at the contact point than retainers formed by ETFE or PFA or a resin composition containing ETFE or PFA as a main component. Accordingly, the rolling bearing having such a retainer (rolling bearing according to the second aspect) exhibits a longer life under a high load or at a high rotary speed than rolling bearings comprising a retainer formed by ETFE or PFA or a resin composition comprising ETFE or PFA as a main component.
Further, the melting point of PVdF is from about 160xc2x0 C. to 180xc2x0 C. and thus is remarkably lower than that of ETFE or PFA. Thus, the mold temperature and the resin temperature at which the resin composition is melt-molded can be lowered from that required for ETFE or PFA. In this arrangement, the production of highly corrosive fluorine gas can be inhibited. Further, the thermal deformation of the mold can be inhibited, making it possible to prolong the life of the mold as compared with the case where ETFE or PFA is used.
The retainer according to the first aspect is formed by melt-molding PVdF or a resin composition containing PVdF as a main component and thus can be obtained at a higher productivity than retainers containing PTFE as a main component. The melt-molding of the resin composition can be accomplished by a known molding method such as injection molding, compression molding and transfer molding. In particular, injection molding, which gives an excellent productivity, can be employed to reduce the production cost of retainer to advantage.
The rolling bearing according to the second aspect can be kept self-lubricating over an extended period of time even if no lubricants or greases are supplied into the interior of the bearing because when the retainer mainly composed of PVdF comes in frictional contact with the rolling body, PVdF is transferred to the rolling body to for a thin PVdF lubricant film thereon. In other words, since the rolling bearing according to the second aspect requires no lubricants or greases to be supplied in the interior thereof, the external atmosphere cannot be stained by lubricant or grease.
The rolling bearing according to the third aspect comprising an inner ring, an outer ring, a rolling body and a retainer. The retainer is obtained by melt-molding a resin composition containing a melt-moldable heat-resistant resin blended with a liquid crystal polymer and a solid lubricant.
The resin composition contains a melt-moldable heat-resistant resin as a main component (in an amount of, e.g., not less than 50% by weight based on the total weight of the resin composition) and a liquid crystal polymer and a solid lubricant as essential components besides the main component. The resin composition exhibits a raised fluidity during melt molding due to the incorporation of liquid crystal polymer and thus can be fairly melt-molded to form a self-lubricating retainer having an excellent heat resistance. The retainer thus obtained by melt-molding the resin composition can hardly be cracked at the tip or curved portion such as lip for holding rolling bodies. Further, the resin composition thus melted can solidify at a raised rate when cooled in the mold and thus can hardly be burred. Moreover, when injection molding is used as melt molding, the resin composition can be molded at a relatively low injection pressure.
Further, when the resin composition is kneaded or molded, the heat-resistant resin and the liquid crystal polymer undergo fibrillation to exert a self-reinforcing effect. Thus, the retainer obtained by melt-molding the resin composition has a sufficient mechanical strength required for retainer.
Thus, the retainer constituting the rolling bearing of the present invention not only has self-lubricating properties and excellent heat resistance but also is a product of high precision melt molding. In this arrangement, the rolling bearing according to the present invention can be kept fairly lubricated scattering little particles to the exterior of the bearing even in a high temperature atmosphere over an extended period of time and can be produced at low cost.